1. Field of the Invention
The present invention relates to an antenna technology that can be used for information communication apparatuses including mobile communication apparatuses, small information terminals and other radio apparatuses. More particularly, the present invention relates to an antenna that can perform transmitting/receiving at wide bands, can be used in low frequency bands, and that is small and light. In addition, the present invention relates to an information communication apparatus using the antenna.
2. Description of the Related Art
In recent years, products that utilize radio technology are being widely used as the development of the radio communication technology has surged forward. As for the radio apparatus such as the mobile communication terminal, it is strongly required to downsize the antenna as the radio apparatus is downsized.
In addition, development of a wide-band and small antenna is expected for supporting plural communication schemes and for supporting wide band transmission such as UWB (Ultra Wide Band).
FIG. 1 shows a configuration of a conventional discone antenna. The discone antenna is a monopole antenna including a disc-like base plate (grounded conductor) 101 and a cone-like radiating element 102.
An ideal discone antenna is one that has an infinite size and does not have dependency on frequency. However, since an actual discone antenna has a finite size, an upper limit of an operating wavelength is limited to about four times the length of the radiating element.
A conventional example of such an in-horizontal-plane nondirectional antenna formed with the grounded conductor and the radiating element is described in the following, in which the following example is modified for realizing wide-band communication.
FIGS. 2A and 2B show an antenna disclosed in Japanese Laid-Open Patent application No. 09-083238 (Patent document 1), in which FIG. 2A shows a perspective view of the antenna and FIG. 2B shows a side elevation view of the antenna.
This antenna includes a skirt part 110 and a top load part 120. The skirt part 110 includes a cone base 111 in which spiral conductive elements 112 (112a, 112b) are formed on the outside surface of the cone base 111. The top load part 120 includes a plane base 121 placed near the top of the skirt part 110 in which a meander-like conductive element 122 is formed on the surface of the plane base 121. Power is supplied from a feeder 130.
In the antenna, the shape of the meander-like conductive element 122 on the plane base 121 is relatively wide beltlike. In addition, the antenna can realize multiple resonance due to existence of plural meander lines. Thus, the antenna is configured to perform wide-band communication.
In addition, the antenna can realize an electrical length longer than its appearance due to the spiral conductive elements 112 (112a, 112b) formed on the skirt part 110. Thus, the size of the antenna can be reduced compared with a conventional discone antenna.
However, as for this antenna, it is necessary to form the meander-like and spiral-like conductive patterns on the base, and it is necessary to increase density of the conductive patterns as wider-band is required. Therefore, there is a problem in that the structure of the antenna is complicated.
FIGS. 3A and 3B show an antenna disclosed in Japanese Laid-Open Patent application No. 09-153727 (Patent document 2), in which FIG. 3A shows a front view of the antenna and FIG. 3B shows a bottom view of the antenna.
This antenna includes a conductor 144 that is a radiating element and a metal plane base plate 143 that is a reflector plate. The outside surface of the radiating element is shaped like a body of semiellipse revolution or shaped like a semiround body, and the top of the conductor 144 is attached to the plane base plate 143 using a coaxial connector 142.
By adopting the body of semiellipse revolution or the semiround body as the shape of the radiating element, this antenna is downsized and is adapted to wide-band communication. However, for realizing wider-band communication so as to be able to use lower frequency, the size of the antenna needs to be increased.
As mentioned above, according to the conventional antenna, the antenna structure is complicated in order to realize wide-band communication, and the size of the antenna needs to be increased in order to adapt the antenna to lower frequency band.